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# Antiparticles

by Ron Kurtus (revised 26 August 2014)

Antiparticles are subatomic particles that have the same mass and characteristics of a standard particle, except that they have the opposite electric charge. For example, an antiproton is just like a proton, except that it has a negative (−) electric charge instead of a positive (+) charge.

Antiparticles can be created from high-energy collisions and through radioactive decay. When antiparticles come in contact with their counterpart particles of matter, they often annihilate each other and give off energy in the form of radiation. The PET scan is an application of antiparticles in the area of health care.

Questions you may have include:

• What are the major antiparticles?
• How are they created and destroyed?
• Is there a practical use for antiparticles?

This lesson will answer those questions. Useful tool: Units Conversion

## Antiparticles

Subatomic antiparticles include the positron, antiquark, antiproton and antineutron, as well as other minor particles such as the antineutrino.

### Positron

The positron is the antiparticle of an electron.

The electron is considered a fundamental subatomic particle. That means it is not made up of smaller particles. The electron has a negative (−) electrical charge.

A positron is the name of an antielectron. It has the same mass as an electron, except that it has a positive (+) electrical charge.

### Antiquarks

Another fundamental subatomic particle is the quark. There are two types of quarks, the up quark and down quark.

The up quark has an electrical charge of (+2/3). The anti-up quark has an electrical charge of (-2/3).

The down quark has an electrical charge of (-1/3). The anti-down quark has an electrical charge of (+1/3).

NOTE: When electrical charges were defined, the charge of an electron was assigned as (+1) and that of a proton as (-1). That has already caused confusion in the flow of electricity. But now it is found that quarks have a fraction of that unit charge. Thus, we see a quark has electrical charges of 1/3 and 2/3, which are not very intuitive.

The protons and neutrons that make up the nucleus of an atom were once thought of as fundamental subatomic particles. It has been shown that they are actually made up of quarks.

(See Fundamental Subatomic Particles for more information.)

### Antiproton

The proton is made up of two up quarks and one down quark. The electrical charge of the proton is then: (+2/3) + (+2/3) + (-1/3) = (+1).

The antiproton is made up of two up antiquarks and one down antiquark. The electrical charge of the antiproton is then: (-2/3) + (-2/3) + (+1/3) = (-1).

### Antineutron

The neutron is made up of one up quark and two down quarks. The resulting electrical charge of the neutron is: (+2/3) + (-1/3) + (-1/3) = (0).

The antineutron is made up of one anti-up quark and two anti-down quarks. The resulting electrical charge of the neutron is: (-2/3) + (+1/3) + (+1/3) = (0).

## Formation and destruction

Antiparticles occur in nature or can be created with nuclear accelerators. Collisions between antiparticles and particles of matter result in energy being given off in the form of radiation.

### Creation of antiparticles

Radioactive decay of an unstable isotope can create antiparticles in some situations. For example, Carbon-11 is a radioactive isotope of carbon that decays into Boron-11 by giving off a positron.

Antiparticles are formed naturally in space and on the various suns or stars in the Universe as a result of high-energy particle collisions. High-energy cosmic rays from space strike atoms in the atmosphere and create antiparticles. They quickly collide with matter articles and annihilate.

High-powered nuclear accelerators also are used to create extremely small amounts of antiparticles.

### Collision of particle and antiparticle

When one antiparticle interacts with an equivalent particle, an enormous amount of energy is given off in the form of electromagnetic radiation or its equivalent photon particles. The totality of the mass of the two particles is converted into electromagnetic energy through Einstein's famous E = mc² equation. The electromagnetic radiation consists of gamma rays, which have a shorter wavelength and higher energy than x-rays.

The radiation consists photons that are moving in opposite directions at the speed of light. They move in the opposite direction to fulfill the Law of Conservation of Momentum, resulting in zero change in momentum. As more antiparticles interact, the photons become considered as a waveform or gamma rays.

## Uses of antiparticles

The most common use of antiparticles is in Positron Emission Tomography (PET), which is a medical scan of body areas that can produce 3-dimensional pictures of their functions.

The way this works is that radioactive isotopes, such as Carbon-11 or Oxygen-15 are mixed with in a special solution and injected into the person's blood stream. The solution then collects in an area of interest, such as a suspected cancer tumor.

(See Isotopes in the Chemistry lesson for more information.)

The isotopes have a half-life of several minutes, meaning that 1/2 of them will decay within that time. These specific radioactive isotopes give off a positron when they decay. When the positron collides with an electron in the body, they annihilate each other, and give off high energy gamma rays in opposite directions. By collecting data from detected gamma rays—which are similar to high energy x-rays—a computerized images of the tumor area can be created.

## Summary

Antiparticles have the same mass and characteristics of a standard subatomic particles, except that they have the opposite electric charge. The positron, antiproton and antineutron are common antiparticles. They are created from high-energy collisions and from radioactive decay.

When antiparticles come in contact with particles of matter, they give off energy in the form of radiation. A PET scan is an application of antiparticles in the area of health care.

Examine the impossible

## Resources and references

Ron Kurtus' Credentials

### Websites

Antiparticles - HyperPhysics

Antiparticle - Wikipedia

Particle/Anti-Particle Annihilation - Matt Strassler, Physicist

Electronâ€“positron annihilation - Wikipedia

Particleâ€“antiparticle annihilation - Wikipedia

Antimatter - CERN Partical Accelerator

Anitmatter - Wikipedia

The five greatest mysteries of antimatter - New Scientist Magazine

Physics Resources

## Questions and comments

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